This invention relates to an emitter follower type single-ended push pull circuit of the non-cutoff class "B" type.
Emitter follower type circuits have been generally of class "B" efficiency and are inevitably of a vertical transfer characteristic. Conventional circuits of this class have a tendency to generate switching distortion when one transistor is on the the other transistor is cut off. To eliminate this defect and to make the "B" class circuit a non-cutoff type, of late, it has been a matter of course to use a servo circuit for providing a constant idle current flowing through the transistors to thereby prevent cut-off at all times. In this manner, switching distortion may be surely remedied; however, no means has been adopted to cope with the current distortion derived from the non-linearity of the current transfer characteristic inherent in the transistor and the voltage distortion caused by an exponential transfer characteristic.
Further, an idle current involves the disadvantage of thermal runaway if temperature is not compensated in the case of a bipolar transistor. In the past, there has been no means for controlling the idle current to a constant value, so that the idle current value is consequently varied depending upon the presence of a signal or the extent of the ambient temperature. This is not desirable since the operation point may be changed for a given time duration irrespective of the presence of the signal.
In addition, since such temperature compensation is very severe and, specifically, since the conventional circuit of the non-cutoff class "B" type makes use of positive feedback, the instability of the idle current is enhanced. It is very difficult to design a circuit capable of performing complete temperature compensation simultaneously with other design requirements.
FIG. 1 shows the basic structure of the conventional single-ended push pull circuit (hereinafter referred to as a "SEPP" circuit) of the non-cutoff class "B" type, in which letters A.sub.1 and A.sub.2 designate error amplifiers, each of which has a gain of 1 or less. Letters B.sub.1, B.sub.2 designate voltage generator circuits, that is, voltage summing devices, and C indicates an input signal source, V.sub.B being a bias current source for transistors Q.sub.1 and Q.sub.2.
In FIG. 1, designated by iE.sub.1 and iE.sub.2 are idle currents Id when there is no current at the input IN. I.sub.B1 and I.sub.B2 are supplied from a power source V.sub.B. Now, if the base emitter voltage is denoted as V.sub.BE, and the emitter resistance is expressed by R.sub.E, the following equation will hold. ##EQU1##
An input signal current i.sub.i flows to increase i.sub.E1, whereupon the current-amplification factor of a transistor Q.sub.1 is expressed by h.sub.fel to obtain the following equation. EQU i.sub.E1 =h.sub.fe1 .multidot.i.sub.i
Current i.sub.E1 allows resistance R.sub.E to have a voltage generated across opposite ends thereof, whereby the input voltage V.sub.i1 of amplifier A.sub.1 may be expressed as follows: EQU V.sub.i1 =(V.sub.BE -V.sub.B)+i.sub.E1 R.sub.E =(V.sub.BE -V.sub.B)+h.sub.fel .multidot.i.sub.i R.sub.E
This voltage, when no amplifier A.sub.1 is provided, allows transistor Q.sub.2 to be reversely biased and cut off. In contrast, the gain of the amplifier A.sub.1 is set at 1, and the voltage V.sub.i1 intact is positively fedback to the base of the transistor Q.sub.1 to thereby always admit of a constant flow of idling current I.sub.d. Even if the input signal current i.sub.i is inverted allowing the transistor Q.sub.2 to be turned "on", entirely the same actions are performed to establish the SEPP circuit without cutting off the transistor Q.sub.1.
FIG. 2 is a representation explanatory of the current transfer characteristic in response to the input signal current i.sub.i in the circuit shown in FIG. 1. In general, the transistor tends to rapidly lower the current-amplification factor h.sub.fe when its emitter current is increased, to render its resultant characteristic considerably non-linear as shown, thus generating current distortion. As afore-mentioned, when the gain of the amplifiers A.sub.1 and A.sub.2 is set at 1, the positive feedback ratio becomes 100% of the idle current I.sub.d, causing the effect of the resistor R.sub.E, i.e., stabilization to be completely lost. That is, R.sub.E in equation (1) can be considered as zero. Accordingly, the idle current I.sub.d is not stabilized, to cause oscillation. In practice, the gain of the amplifiers A.sub.1 and A.sub.2 is less than 1, however, the idle current is nonetheless rendered extremely unstable.
When the circuit is not constituted so as to be of the non-cutoff class "B" type and is of a constant voltage drive to eliminate only distortions caused from the current transfer characteristics thereof, i.e., when the amplifiers A.sub.1 and A.sub.2 in FIG. 1 are eliminated and a constant voltage source is employed as an input signal source C, the transfer characteristic is as shown in FIG. 3. Even, under such circumstances, distortion caused by the exponential transfer characteristic of the transistor is still existent.
In the conventional SEPP circuit of the class "B" type to which a constant voltage drive method is applied, (1) distortion derived from an exponential and functional transfer characteristic and switching distortion caused by the on-off action of an output transistor are generated. Even if the SEPP circuit of the non-cutoff class "B" type relies on a constant-current drive method, (2) the occurrence of distortion due to the current transfer characteristic has been unavoidable. Irrespective of the drive method, (3) temperature compensation for the idle current is required, however, it is impossible to completely effect temperature compensation. (4) It takes as much as several tens of minutes or more from the time when the power source turns on until the idle current is constant. (5) The idle current fluctuates according to the presence of a signal, and the magnitude of the idle current after a large signal is applied greatly departs from the set point. (6) The performance points are unstable because of reasons (3), (4), and (5), that is, they are variable according to the ambient temperature and the presence or absence of a signal.